1. Field of the Invention
This invention relates to reinforced glass fibers for optical fransmission to be adapted for use in optical communications (hereunder referred to as optical fibers).
2. Description of the Prior Art
Since the optical fibers should be 200 mm or less in diameter to retain their flexibility and they are made of a fragile material, it is almost impossible to use them as a transmission line without any protection in view of their mechanical strength.
In addition, it is well known that, as an inherent property, glass has a tendency to lose its strength with time due to the influence of moisture and other factors. Therefore, several prior art techniques have been proposed for covering an optical fiber with a protective coat of plastics or other suitable materials so as to provide the optical fiber with a desired initial strength and a strength that withstands extended use. For example, a coated optical fiber obtained by the method disclosed in Japanese Patent Application (OPI) No. 125754/75 which comprises coating an optical fiber with a thermosetting resin composition (generally referred to as a primary coat) and baking the resin coating and further providing thereon a coating of a melt-extruded thermoplastic resin composition (a secondary coat), possesses satisfactory strength and weatherability sufficient to withstand extended use. Also, as disclosed in Japanese Patent Application (OPI) No. 100734/76, it is known that a spun optical fiber, prior to its contact with another solid object, can be coated with a resin composition which is then baked to provide the fiber with a strength not substantially lower than the virgin strength of the glass.
On the other hand, a stress absorbing layer of small Young's modulus has been provided between the primary coat of thermosetting resin and the secondary coat of thermoplastic resin to eliminate the increased transmission loss due to a so-called "microbending phenomenon" which occurs when an optical fiber is repetitiously bent in small radius. Examples of the materials which have been proposed for the stress absorbing layer are silicone resin, urethane rubber, butadiene rubber, ethylene-propylene rubber and foamed plastics. Of these materials, the silicone resin has been used widely because of its high processability, good curability and weatherability. The term "silicone resin" as used herein refers to a two-part room temperature vulcanizing resin (RTV) which is generally referred to as a curable organopolysiloxane composition.
Of various organopolysiloxanes, dimethyl polysiloxane composition which is generally commercially available has a refractive index of about 1.40 which is lower than the refractive index of glass. Therefore, if dimethyl polysiloxane is directly coated on an optical fiber and then baked, the resulting glass fiber has the following disadvantages.
When an optical fiber having a distribution of refractive index as illustrated in accompanying FIG. 1 is coated with a layer of an organopolysiloxane composition having a refractive index of about 1.40, the resulting transmission system as shown in FIG. 2 comprises the desired transmission system having I as the core and another transmission system having II as the core and the organopolysiloxane as the cladding. Since the transmission system having II as the core suffers an optically higher loss than the system having I as the core, light excited in the region II will be damped in a distance of about ten-odd meters. As a result, estimation of optical transmission in terms of the ratio of the optical output as a point one to two meters away from the incident end (P.sub.in) to the optical output at a point several hundred to thousand meters away from the incident end (P.sub.out) cannot be made correctly because P.sub.in includes the optical output from the transmission system having II as the core and is therefore over estimated. The second transmission system (or cladding transmission system) having the core of II is undesirable and makes the correct measurement of transmission loss difficult.
In addition, if the light loss in the region of II is relatively low, light excited in the region II will reach the receiving end. On the other hand, the core of an optical fiber is generally prepared by controlling the distribution of its refractive index to obtain a desired level of transmission band (or base-band-frequency characteristics) which is one element of its transmission characteristic. Therefore, emergence of light excited in the region II at the receiving end will seriously degrade the transmission band of the fiber.
It is to be understood that while FIGS. 1 to 3 illustrate examples of the distribution of refractive index of an optical fiber to which this invention is applicable, optical fibers having other distributions of refractive index are included within the scope of this invention.